1. Field of the Invention
The present invention relates to a recording medium which enables high-density recording and a method of reproducing information recorded therein and more particularly, to a recording medium having a constant birefringent property and an alterable photochromic property.
2. Background of the Invention
In recent years, an organic photochromic material of a photon mode has been studied as a material for use as a recording layer. When the photochromic material is exposed to light of a prescribed first wavelength, a photochemical reaction changes the molecular structure of the material. The changed molecular structure returns to its original structure when the material is exposed to light of a second wavelength. In response to such change in the molecular structure, absorptiveness of the material is also greatly changed with respect to light beams of prescribed wavelengths.
A photochromic material having such a property can be used as a recording layer in a recording medium in order to record information using a light beam having the aforementioned first wavelength. Further, the information can be reproduced through a light beam having the aforementioned second wavelength.
When such a reproducing method is employed, however, a previously recorded portion of the recording layer absorbs the reproducing beam, and hence the molecular structure of the previously recorded portion is unfortunately changed to that of an unrecorded state.
Japanese Patent Laying-Open Gazette 63-259850 discloses a reproducing method that utilizes the optical rotatory power of the photochromic material to record information. The term "optical rotatory power" indicates such a phenomenon wherein a prescribed material rotates the plane of polarization of a linearly polarized light beam which is introduced into and transmitted through the material. In the aforementioned photochromic material, presence/absence of such optical rotatory power determines the recorded/unrecorded state. Thus, it is possible to read information by a reader which is provided with means for detecting such rotation of the polarization azimuth angle of a linearly polarized light beam. According to this method, further, it is possible to select a wavelength for the reproducing beam which is not absorbed by the recording layer. Such a selection is possible because optical rotatory power is also present in a region out of the band of absorbed wavelengths. Thus, it is possible to prevent the aforementioned erasing of information caused by the reproducing beam.
FIG. 10 is a sectional view showing a conventional recording medium which is disclosed in Japanese Patent Laying-Open Gazette No. 63-259850. Referring to FIG. 10, a recording layer 102 is formed on a transparent substrate 101, and a reflecting layer 103 is formed on the recording layer 102. This conventional recording medium records information by exposing a portion of the recording layer 102 to a light beam having a specific wavelength and changing the molecular structure of the exposed portion.
In such a reflection type recording medium, however, it is impossible, in principle, to reproduce information that has previously been recorded using changes in the optical rotatory power of the medium. The reason for this is as follows:
FIGS. 11A to 11D illustrate states of rotation of the azimuth angle of the polarization of a linearly polarized light beam which is introduced into the conventional recording medium shown in FIG. 10.
FIG. 11A shows the plane of polarization of the beam which is incident upon the recording layer. FIG. 11B shows the plane of polarization of the beam, transmitted through the recording layer, which is on the point of being reflected by the reflecting layer. As shown in FIG. 11B, the plane of polarization of the beam is rotated clockwise by an angle .delta. as viewed from the substrate. FIG. 11C shows the plane of polarization of the beam just reflected by the reflecting layer. As shown in FIG. 11C, the plane of polarization of the beam is reflected in the state rotated by the angle .delta.. FIG. 11D shows the plane of polarization of the beam retransmitted through the recording layer. When the beam is thus retransmitted through the recording layer, its plane of polarization is rotated counterclockwise by the angle .delta. as viewed from the substrate, since the reflected beam advances in a direction opposite to that of incidence. Consequently, the plane of polarization of the reproducing beam reflected from the recording medium returns to the state the beam had upon incidence, as shown in FIG. 11D. In the conventional reflection type recording medium, therefore, it has been impossible, in principle, to reproduce information that has previously been recorded by altering the optical rotatory power of the medium.
Also when a circularly or elliptically polarized light beam is employed, no change is caused in the polarized state of the reflected beam in response to the state of the recording layer. Hence it is impossible to reproduce information that has been recorded by changing the optical rotatory power of the recording layer. In the recording medium of the conventional structure as shown in FIG. 10, therefore, no change is caused in the polarized state of the beam reflected from the recording medium, even if the recording layer has optical rotatory power. Thus, it has been impossible to read information that has been recorded by changing the optical rotatory power of a material.